Sintering alloy

ABSTRACT

A sintering alloy powder for use in forming products by powder metallurgical techniques contains from 2 percent to 13 percent chromium, optionally up to 5 percent copper and up to 2.5 percent carbon and the remainder except for impurities iron. The alloy powder is preferably made by annealing an iron-chromium alloy powder having a particle size less than 150 Mu m for 2 hours at a temperature of between 850* and 950*C and subsequently mixing the annealed poweder with iron powder with a particle size less than 400 Mu m to adjust the chromium content to the desired value. Alternatively in iron-chromium alloy powder with a particle size less than 150 Mu m is mixed with a fine iron powder with a particle size less than 40 Mu m, the mixture is annealed with exclusion of air for 2 hours at a temperature of from 850* to 950*C and after this the annealed powder is reduced in particle size and is mixed with iron powder to give the desired chromium content.

ited tet H 1 Thiimrnler et a1.

[ SlN'HlElRING ALLOY 22 Filed: Dec. 16, 1971 211 Appl. No.: 98,965

[30] Foreign Application Priority Data Dec. 20, 1970 Germany 1963860 [52] US. Cl 75/.5 R, 75/.5 AA, 75/.5 BA [51] Int. Cl 1322f 9/00, C22C 33/02 [58] Field of Search 75/.5 BA, .5 BC, .5 AA, I 75/.5 AC, .5 R

[56] References Cited UNITED STATES PATENTS 3,293,007 12/1966 Wukusick 75/.5 BA 3,556,780 11/1971 l-loltz, Jr. 75/.5 BC 3,704,115 11/1972 Wastenson et a1. 75/.5 BC

Primary ExaminerW. W. Stallard Attorney, Agent, or FirmToren, McGeady & Stanger cc. HQ, 1974 5 7 STRACT A sintering alloy powder for use in forming products by powder metallurgical techniques contains from 2 percent to 13 percent chromium, optionally up to 5 percent copper and up to 2.5 percent carbon and the remainder except for impurities iron. The alloy powder is preferably made by annealing an iron-chromium alloy powder having a particle size less than 150 pm for 2 hours at a temperature of between 850 and 950C and subsequently mixing the annealed poweder with iron powder with a particle size less than 400 um to adjust the chromium content to the desired value. Alternatively in iron-chromium alloy powder with a particle size less than 150 pm is mixed with a fine iron powder with a particle size less than 40 pm, the mixture is annealed with exclusion of air for 2 hours at a temperature of from 850 to 950C and after this the annealed powder is reduced in particle size and is mixed with iron powder to give the desired chromium content.

3 Claims, N0 Drawings This invention relates to sintering alloys containing a major proportion of iron, together with chromium and also, if desired, copper. These alloys are highly suitable for manufacturing metal products by powder metallurgical processes.

One of the most important fields of application for powder metallurgy is in the manufacture of precision parts from metallurgical powders containing mainly iron. A variety of methods are used, the simplest of which may be called the single pressing process. This involves essentially four process stages. In the first stage the metal powder is prepared to form a freely flowing and easily pressable powder. In the second stage the powder is given a preliminary compaction by a mechanical or hydraulic press which is usually an automatic press, using shaped tools, to form a stable blank. In the third stage of the process the resulting blank is sintered in an atmosphere of inert gas in a sintering furnace at temperatures between l000 and I300C for between 30 and 240 minutes. Whereas the initial blank is held together merely by adhesion between the particles, the sintered product has considerable mechanical strength. The density ratio in the product, as well as its physical properties, depends on the sintering temperature and sintering time. During the sintering the dimensions of the blank are somewhat mofidied. The sintered product therefore usually has its dimensions corrected in a fourth stage by a calibration. This gives the product the desired precise dimensions but hardly changes its physical properties.

In this process the costs are determined by the consumption of raw material and power, by the pressure, temperature and time required and by the rate of abrasion of the shaping tools. The field of application for the resulting sintered products depends on its physical properties, its dimensional accuracy and on the economic, efficiency of the manufacturing process. Sintered products can be made economically and with high precision by this process from unalloyed iron powders, but their field of application is limited due to their comparatively poor physical properties. A sintered product made of iron powder by the process described above, that is to say by the single pressing process, has a tensile strength of between 22 and 24 kp/mm and an elongation at rupture of between 18 percent and 20 percent.

To obtain a higher tensile strength it is necessary to resort to alloying during the first stage of the process by initially preparing a mixed metal powder. In producing mechanical parts for the manufacturing industries sintering alloys made of iron and copper are therefore often used. The inital powder is usually a mixture of iron powder and copper powder. The two powders are mixed together in specified proportions. In the second stage of the process the mixture of powders is compacted under a pressure which does not exceed 6 Mplcm to produce a blank which is subsequently sintered in an atmosphere of inert gas at a temperature of between 1000 and l300C. The resulting sintered product has a tensile strength of between 36 and 38 kp/mm, depending on the copper content, and an elongation at rupture of between 2 and percent.

In many fields of application this tensile strength is insufficient. The powder metallurgical industry, which is concerned with the production of parts for the manufacturing industries, is therefore constantly striving to find alloys which, processed under the pressure and sintering conditions mentioned above, are capable of producing products having better mechanical properties, particularly a higher tensile strength or, at equal tensile strength a greater elongation at rupture.

The object of the present invention is to provide a sintering alloy capable of satisfying the requirements in regard to these two main physical properties, and which therefore has a wider field of application.

In particular products made from alloys in accordance with the invention have higher tensile strengths compared with products made from conventional ironcopper sintering alloys, without any less elongation at rupture than that of products made from unalloyed sintering iron powder.

To this end, according to this invention, a sintering alloy powder contains a major proportion of iron, and from 2 to 13 percent chromium together with impurities. The particle size of the powder is preferably less than um. I

The sintering alloy powder may also contain up to 5 percent of copper, there then being preferably at least 0.5 percent of copper present.

The products made from the binary iron-chromium sintering alloy powder in accordance with the invention not only have higher tensile strengths than products made from unalloyed sintering iron powder but also have a smaller reduction in elongation at rupture than products made from conventional iron-copper sintering alloys.

Products made from the alloy powders in accordance with the invention which also contain copper have the further advantage that the tensile strength is increased, compared with those made from conventional ironcopper alloys. Binary sintering alloys of the ironchromium system with chromium contents within the range covered by the present invention, and ternary sintering alloys of the iron-chromium-copper system with compositions within the range covered by the invention have not hitherto been used in powder metallurgical processes and neither has their use been suggested been described in the literature. Apparently these alloys have not aroused technical interest in this field, because it has not hitherto been possible to make sintering powders with high chromium contents, having sufficiently good pressing and sintering properties.

The invention provides sintering alloy powders from which sintered products can be made which have the physical properties mentioned above, the powders themselves having excellent pressing and sintering properties. i

The sintering alloy powders are, in accordance with another aspect of the invention, preferably made by either one of the following processes:-

In one process an iron-chromium. alloy powder with a particle size less than 150 ,u.m is annealed for 2 hours at a temperature of between 850 and 950C, after which the annealed powder is mixed with iron powder with a particle size less than 400 am. to adjust the chromium content to the desired value.

In the second process an iron-chromium alloy powder with a particle size less than 150 pm is mixed with a fine iron powder with a particle size less than 40 ,um, the mixture is annealed with exclusion of air for 2 hours at a temperature of from 850 to 950C, whereupon the annealed powder is reduced in particle size and is mixed with iron powder to give the desired chromium content. Preferably the iron chromium alloy powder is a ferro-chrome with a chromium content of between 35 and 50 percent by weight. If desired the annealed powder may also be mixed with copper powder with a particle size less than 150 um.

The iron powder with which the annealed powder is mixed can consist of pure iron, for example electrolytic iron, or it can be an iron containing small quantities of other elements, for example a pig iron powder. The small particle size of the powder can be obtained by means of the usual size reducing processes, for example by spraying or by reduction from iron compounds. Both pig iron sprayed powder and reduced powder is suitable for the process according to the invention.

With the second process, either a mixture of ferrochrome powder with iron powder, the mixture having the finally desired 2 to 13 percent chromium content may be used, or alternatively a powder mixture containing more chromium, for example between 18 and 30 percent chromium, can be annealed, the chromium concentration being subsequently brought down to between 2 and 13 percent by adding further iron powder.

To make a sintered product from a powdered alloy in accordance with the invention, the powdered alloy is pressed to make a shaped blank and the blank is then sintered at a temperature of between 1000 and l300C preferably between l200 and l300C.

The resulting sintered products have chromium contents between 2 and 13 percent, depending on the initial powder mixture, and if desired copper contents between 0.5 and percent.

The sintering powder prepared in accordance with the invention has very good pressing properties, corresponding approximately to those of pig iron sprayed powders and reduction powders, as is shown in the following Table 1.

Table 2 shows the density ratios obtained when the powder contains 9 percent of chromium, the Table showing the values for four different mixtures. Powder No. 1 is a mixture of annealed ferrochrome powder and electrolytic iron powder. Powder No. 2 consists of ferrochrome powder and iron powder annealed together. Powder No. 3 is a mixture of ferrochrome powder and iron reduction powder. Powder No. 4 is a mixture of ferrochrome powder and pig iron sprayed powder together with an addition of 1 percent Zn stearate as a lubricant.

TABLE 1 Relative density 1%! Iron powder Cr (7r) m 2 Mp/cm 4 Mp/cm 6 Mp/cm 0 0.78 68.5 78.5 85.0 2 0.78 68.0 78.0 84.0 Spray 4 0.74 67.5 77.5 83.5 powder 6 0.72 67.0 77.0 83.0 R2 150 9 0.67 66.6 76.0 82.2 13 0.63 65.5 74.5 81.0 18 0.62 64.5 73.6 80.0 0 0.77 68.0 78.0 84.5 2 0.77 67.0 77.0 84.0 Reduction 4 0.77 67.0 77.0 84.0 powder 6 0.77 67.0 77.0 84.0 MH 100/24 9 0.77 67.0 77.0 84.0 13 0.77 67.0 77.0 84.0 18 0.77 67.0 77.0 84.0

TABLE 2 Relative densit Powder 2 Mp/cm 4 Mp/cm 6 Mp/cm In a preferred example of the invention, using either a binary iron-chromium alloy powder containing from 2 to 13 percent of chromium, or a ternary ironchromium-copper alloy containing from 2 to 13 percent of chromium and from 0.5 to 5 percent of copper, the tensile strength of the product is still further increased by sintering in a carbon-increasing atmosphere. This can be obtained by adding to the sintering alloy powder a substance containing carbon, for example graphite or a lubricant which decomposes at the sintering temperature. Particularly favourable results are obtained with a carbon content of between 0.05 to 0.3 percent. It has been found that this carbon content also results in a very high elongation at rupture.

In the manufacture of sintered products'from the alloy powder, blanks are made by compressing the alloy powder in a press under a pressure of from 4 to 7 Mplcm giving a blank with a density of between 6.4 and 6.9 g/cm.

The blank is sintered at a temperture of 1000 to 1300C and the resulting sintered product can if desired be calibrated to give it precise dimensions. Using this method, starting with a mixture of electrolytic iron powder HVA-Star and a fine annealed ferrochrome powder the values shown in Table 3 were obtained for tensile strength, elongation at rupture and Brinell hardness, the values depending on the chromium content and the sintering temperature. All the values shown in the Table relate to a density of 6.7 g/cm in the blank. Sintering was continued in all cases for 2 hours in an atmosphere of hydrogen. A higher blank density gives somewhat higher values, and a lower blank density somewhat lower values.

From Table 3 it will be seen that tensile strength, and even more so elongation at rupture, increases with increasing sintering temperature. To obtain a particularly high tensile strength and a particularly good combination of tensile strength with elongation at rupture the best sintering temperature is between l200 and 1300C.

Comparing the results shown in Table 3 with those published in the literature for tensile strengths of sintered iron-copper products, the following picture is obtained:

An iron-copper sintering material containing 3 percent chromium, using electrolytic iron powder, compacted to a blank density of 6.7 g/cm was sintered at l200C. The sintered product showed a tensile strength of 28 kp/mm and an elongation at rupture of 6 percent. On the other hand an iron-chromium sintering, material containing 4 percent chromium, using electrolytic iron powder, compacted to a blank density of 6.7 g/cm was sintered at l200C. In this case the sintered product showed a tensile strength of 29 kp/mm and an elongation at rupture of 12 percent.

The following is a comparision between, on the one hand, alloys containing 4.5 percent copper, which is. usually the highest copper content, and, secondly, alloys containing 4 percent of chromium and, thirdly, alloys containing 6 percent chromium, operating conditions being otherwise the same in all cases:

lron-copper Tensile strength 30 kp/mm 4.5% Copper Elongation at rupture 2.2%

Iron-chromium Tensile strength 29 kp/mm 4.0% chromium Elongation at rupture 12% Iron-chromium Tensile strength 34 kp/mm 6.0% chromium Elongation at rupture 9.6%.

The superiority of the alloys in accordance with the invention emerges clearly in all cases.

With regard to ternary sintering alloys in accordance with the invention prepared as described above and containing from 0.5 to 5 percent of copper, between 0.5 and 1.2 percent ofa lubricant, for example zinc stearate can with advantage be added to the powder. The resulting mixed powder can be pro-pressed at a pressure of between 4 and 6 Mp/cm to give a blank, which is then sintered at a temperature of from 1000 to I300C. The sintered product is if necessary calibrated to provide it with precise dimensions.

Table 4 shows tensile strengths, elongations at rupture and Brinell hardness of sintered products made in accordance wtih the invention from ternary ironchromium copper sintering alloys with various copper contents. Electrolytic iron powder l-lVA-Star was used for the mixture, and a fine annealed ferrochrome powder. The powder mixture was compressed to a density of 6.7 g/cm in the blank, which was sintered for 2 hours in an atmosphere of hydrogen. The mechanical properties are shown in dependence on the sintering temperature, the chromium content and the copper content.

Table 4 shows, to begin with, that even using comparatively low sintering temperatures quite good tensile strengths are obtained. However both tensile strength and elongation at rupture, increases sharply at sintering temperatures from l200C upwards. lf very high tensile strength and elongation at rupture are required, the sintering temperature is preferably from l200 to 1300C. On the other hand if tensile strength is the most important requirement, elongation at rupture being less important, a sintering temperature between 1050 and I 150C is sufficient.

The following comparison can be made between the tensile strengths of alloys according to the invention and those of the known iron-copper materials. Pressing and sintering conditions are the same in all cases and the same iron powder is used. In all cases the density of the pressed blank is 6.7 g/cm Iron-copper Tensile strength 26.5 kp/mm 2% copper Elongation at rupture 7.9

lron-chromium-copper Tensile strength 42 kp/mm 4% Cr, 2% Cr Elongation at rupture 6.2%

lron-chromium-copper Tensile strength 49 kp/mm 6% Cr, 2% Cr Elongation at rupture 6.0

lron-chromium-copper Tensile strength 53 kp/mm 9% Cr, 2% Cr Elongation at rupture 6% EXAMPLE 1 Ferrochrome powder with a particle size not more than 50 um was annealed for 2 hours at 850C with the exclusion of air in an atmosphere of hydrogen. After cooling, the powder was reduced in particle size and a powder mixture prepared containing 1 percent zinc stearate, 6 percent chromium, 2 percent copper and 91 percent iron. The extra iron, apart from the iron in the ferrochrome, was added to the mixture in the form of electrolytic iron powder.

The powder mixture was pre-pressed under a pressure of 6 Mp/cm to form a blank. The blank had a density of 6.8 g/cm. The blank was sintered for 2 hours in an atmosphere of cracked ammonia gas in a closed box sealed against the entry of water vapour and oxygen by means of a getter substance. The sintered product had a tensile strength of 51 Kp/mm and an elongation at rupture of 9 percent.

TABLE 4 2 Cu 4 Cu Cr Sintering Tensile Elongation Brinell Tensile Elongation Brinell content temperature strength hardness strength hardness (C) (kp/mm (71 l-Sd) (kp/mm (kp/mm l-Sd) (kplmm 1050 32 2.8 108 36 2 116 I 36 3.4 110 39 2.6 119 3% Cr I150 42 3.8 114 42 3.0 116 I200 42 6.2 120 43 6.0 I21 I250 44 8.0 128 45 6.3 13% 67rCr [I50 51 2.7 132 5| 2.6 139 1200 49 6.0 136 56 5.0 140 1250 52 8.2 139 52 5.0 158 I050 41 1.8 138 43 1.5 I30 I100 47 2.2 136 48 1.6 142 9 7r Cr I 52 2.6 141 53 2.4 148 1200 53 6.0 Us 54 4.0 147 1250 58 7.4 147 65 4.6

. sintered at l200C for 2 hours in an atmosphere of inert gas containing 70% N and 30% H For sintering the blank was packed in a box sealed against direct penetration by the sintering atmosphere by means of a getter substance. The resulting sintered product hadshrunk during the sintering to a density of 7.0 g/cm. The tensile strength was 30 Kp/mm and the elongation at rupture 11 percent.

EXAMPLE 3 A mixture containing 4 percent of chromium in the form of finely ground ferrochrome powder with a chromium concentration of 42 percent, 4 percent of electrolytic copper powder, 1 percent of zinc stearate and the remainder iron powder in the form of a reduction powder with a maximum particle size of um was pre-pressed under a pressure of 4 Mp/cm to give a blank which had a density of 6.4 g/cm. The blank was sintered in a closed box, in which the blank was protected by a getter substance against penetration by the oxygen and moisture of the sintering atmosphere, in a travelling belt furnace at 1100C. After cooling the product had a tensile strength of 33 Kp/mm and an elongation at rupture of 2.5 percent.

We claim:

I. A sintering alloy powder consisting essentially of about 2 to 13 percent of chromium and about 0.5 to 5 percent of copper, with the balance iron and impurities.

2. A sintering alloy consisting essentially of 2 to 13 percent of chromium,

0 to 5 percent of copper and 0 to 2.5 percent of carbon, with the balance iron and impurities. 3. An alloy as claimed in claim 2, wherein said carbon content is between 0.8 and 2 percent.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,853,537 Dated D cember 10,1974

lnventofls) FRITZ TH{ JMMLER,GERHARD SAPF and MAHMOUD AHMED It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading of the patent:

. Add to the addresses of the inventors:

Mahmoud Ahmed, Masr el Gadida, Gamal Ez Eldin Salamah-Str.

Block 1010/10, Cairo, EYGPT o Signed and Sealed this fourteenth Day Of October 1975 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Atlesting Officer Commissioner ufParents and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 53,537 Dated DCmbT 97 Inventor(5) FRITZ THUMMLER, GERHARD ZAPF and MAI-IMOUD AHTWFD It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading of the patent:

The filing date should read:

[22] Filed: Dec. 16, 1970-- Add to the recitation of the inventors:

--Gerhard Zapf, Krebs5ge/Rhld. Germany-- The Foreign Application Priority Data should read:

. [30] Foreign Application Priority Data Dec. 20 a o 0 I I I I a I a 0 0P Signed and sea led this 1st day of April 1975..

( 3 PAL) Attest:

C. MARSHALL DANN RUTH MASON Commissioner of Patents Attesting Officer and Trademarks FORM "$69) uscoMM-oc 603764 68 9 U.S. GOVERNMENT PRINTING OFFICI I). 0*! 

1. A SINTERING ALLOY POWDER CONSISTING ESSENTIALLY OF ABOUT 2 TO 13 PERCENT OF CHROMIUM AND ABOUT 0.5 TO 5 PERCENT OF COPPER, WITH THE BALANCE IRON AND IMPURITIES.
 2. A sintering alloy consisting essentially of 2 to 13 percent of chromium, 0 to 5 percent of copper and 0 to 2.5 percent of carbon, with the balance iron and impurities.
 3. An alloy as claimed in claim 2, wherein said carbon content is between 0.8 and 2 percent. 